The present invention relates to a method for multicasting a specified file to multiple destinations and, more particularly, to a method for multicasting the specified file to the multiple destinations via wireless links, such as a satellite communication circuit.
As prior art of this technical field, there is proposed an MFTP (Multicast File Transmission Protocol) system in which a transmitting station multicasts a specified file composed of a plurality of blocks, then a receiving station requests retransmission of a DTU (Data Transmission Unit) that could not be received, so that the transmitting station retransmits the requested DTU, thus completing the file transfer (as published in Japanese Pat. Pub. Gazette No. 512726/98).
FIG. 28A depicts a data transmission packet format transmitted in the MFTP system in a case where each file comprises blocks #1, #2, #3 and #4; each block is composed of frames #11 to #1N, and each frame including an MFTP-OH, which is an OH (overhead) for use in the communication path between a transmitting processor and a receiving processor in the MFTP system, and an MFTP-OH (option) for optional use. Each frame including the MFTP-OH (option) constitutes one DTU. This DTU, the METP-OH, a UDP (User Datagram Protocol)-OH and an IP-OH, which is an overhead for IP (Internet Protocol), constitute one MTU (Multicast Transmission Unit). FIG. 28B is a diagrammatic representation of a retransmission-requesting packet format in the MFTP system, which indicates received results for each bit in one DTU. In this example, each cross mark x indicates a result of non-reception.
The conventional MFTP system adopts a method which, during multicast transmission of a file, receives from a plurality of receiving stations response signals which indicate DTUs to be retransmitted. In the MFTP system, since data errors are expressed in the form of a bit map, even where only one DTU is erroneous, it is necessary that the receiving station sends back to the transmitting station a retransmission-requesting packet composed of a bit map (usually, of a size of one MTU (=1500 bytes)) corresponding to one block.
The MFTP system is usually applied to a communication system, whose communication circuits from a plurality of receiving stations to a transmitting station (denoted as “up-link direction”) are dedicatedly allocated to communicate between an individual receiving station and the transmission station, such as conventional telephone circuits. However, since the total amount of data becomes large, the MFTP system has a serious drawback in terms of efficiency in retransmission processing on applying to a communication system, whose circuits in the up-link direction are shared on a multiple access basis, such as satellite communication circuit.